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O n e
Cha p te r

1.15.5 The Inverse Square Law
The inverse square law tells us that the illumination is inversely pro-
portional to the square of the distance between the point source and
the surface, in other words:
E 1 = D 2 2
2
E D
2 1
If you have a fixture that can be treated as a point source, where
the distance from the surface is large, and you measure the illumina-
tion at 20 feet as 2000 Fc at the beam center, then at 40 feet the illumi-
nation is 500 Fc at the beam center.

1.15.6 The Cosine Law
Effective illumination is proportional to the cosine of the angle of
incidence of the light on the surface. It is the angle between the direc-
tion of the light and the perpendicular to the surface (Fig. 1.21).
E = E cosα = E sinβ
2
1
1
Here are a few cases:
When the surface is tilted by an angle of 30º, the illumination is
reduced by a factor of 0.87
45º – 0.71
60º – 0.5

FIGURE 1.21 Source
Illumination at
the “O” point
on surfaces “1”
and “2.”

90°
α
β

E 1
O
E 2